This invention relates to electronic postage meters, and is more particularly directed to an electronic postage meter of the type having a keyboard for the entry of postage to be printed, a display for displaying postage to be printed, and an electronic accounting device incorporating a non-volatile memory for the storage of postage information, the accounting device including a security feature for the prevention of unauthorized erasure of a complete block of the memory.
The invention is well suited to be incorporated into an electronic postage meter of the form disclosed in the U.S. Pat. No. 4,301,507, the disclosed postage meter incorporating an accounting board comprising a non-volatile memory, a microcomputer, and address-generation circuitry responsive to signals of the microcomputer for addressing the non-volatile memory.
The non-volatile memory is used for the permanent recordation of important information such as the amount of postage available, the amount of postage which has been used, and user coding signals which prevent unauthorized access to the postage meter. In particular, it is noted that the non-volatile memory retains the information even in the absence of electric power, such memory being constructed of, typically, MNOS circuitry. As is disclosed in the foregoing U.S. Pat. No. 4,301,507, such memory and the circuitry coupled thereto is enclosed within a metallic shield for the shielding against electromagnetic interference, thereby preventing accidental erasure of data which may be stored in the memory.
A problem arises in that great care must be taken to ensure that there is no alteration of the stored information, particularly no erasure of the entire block of the information, as might occur in the event of the malfunction of a circuit coupled to the memory. While all such circuits, including the foregoing microprocessor and the address-generation circuit well as the memory itself are carefully constructed so as to minimize any chance of failure which might affect the storage of information, nevertheless, it is the nature of electronic circuitry that there is always some chance, albeit a very small chance, that a malfunction could occur and that such malfunction might endanger the storage of the data in the memory.
One particular vehicle by which stored data may be lost arises in the nature of the control circuit for the memory. Typically, such memories are produced commercially as preformed packages with a predesignated arrangement of pins for electrical connection, which pins may be plugged into a socket. In particular, in order to ensure versatile operation of the non-volatile memory, a common address scheme and a common control scheme are provided for all the memories. The control scheme provides for a pair of control lines carrying a two-bit signal for designating four possible functions, namely, read, write, block-erase, and word erase. The selection of the particular function is provided by the control generation circuit in combination with the microcomputer.
The foregoing problem in the maintenance of the security of the stored information is, therefore, directly connected with the generation of the control signals for designating the foregoing four memory functions. In particular, it is noted that while the read and the write functions are continually used during the operation of the accounting board, and that while the word-erase function would be utilized whenever it is desired to update stored data, the block-erase function would never be used to erase the memory in view of its storage of a history file on the use of the postage meter.
It is apparent therefore, that in the event of a malfunction in the microcomputer or in the control generation circuit, which malfunction would inadvertently provide the two-bit control signal designating the block-erase function, then the non-volatile memory would become erased in its entirety. Accordingly, it may be understood that a substantial increase in the security of the operation of the accounting board, particularly with respect to the storage of data in the non-volatile memory, could be obtained if there were provided some form of security device which would prevent the presentation of the unauthorized block-erase signal at the memory.